What is the argument that a jury can determine whether a knife is a deadly instrument based on "common sense"?

Second, the argument is at odds with the majority's characterization of the instructions as "problematic" and its suggestion that they be "modified to avoid these problems in the future." (Maj. opn., ante , 251 Cal.Rptr.3d at p. 384, 447 P.3d at p. 288.) Why alter anything if, as the majority implies, jurors can more or less figure it out on their own? (Id. at p. 384, 447 P.3d at p. 288 [citing People v. Pruett (1997) 57 Cal.App.4th 77, 86, 66 Cal.Rptr.2d 750 for the proposition that jurors can determine whether a knife is a deadly instrument based on "common sense"].) (Indeed, jurors can guess approximate meanings for most legal principles and elements, from robbery to "deadly weapon"; I imagine the majority would not suggest we dispense with those instructions out of an abiding belief that common sense will suffice.)